/Engine/Source/Runtime/Engine/Classes/SparseVolumeTexture/SparseVolumeTexture.h
Multi-phase initialization: CallBeginInitialize(), then callAppendFrame()for each frame to add and then finish initialization with a call toEndInitialize(). The NumExpectedFrames parameter onBeginInitialize()just serves as a potential optimization to reserve memory for the frames to be appended and doesn't need to match the exact number if it is not known at the time.
virtual bool BeginInitialize ( int32 NumExpectedFrames )